Damage-induced SOS mutagenesis occurs transiently as part of the global SOS response to DNA damage. Key participants in this process are the UmuD2C proteins which together with activated RecA (RecA*), and Ssb were previously thought to enable DNA polymerase III holoenzyme to traverse otherwise replication-blocking lesions. A major step towards elucidating the biochemical mechanism of Umu-dependent translesion DNA synthesis was recently achieved with the striking discovery that the UmuD2C complex possesses intrinsic DNA polymerase activity and as a consequence has been called E. coli pol V. The UmuD mutagenesis protein is functionally inactive until it undergoes a RecA-mediated posttranslational cleavage reaction to generate UmuD. Unlike the structurally related LexA protein (which serves as a paradigm for self-cleavage studies), experiments revealed that UmuD cleavage predominantly occurs via an intermolecular reaction Using chimeric proteins, we were able to identify residues within UmuD that are required for efficient intermolecular cleavage. Our data suggests that intermolecular cleavage occurs only when UmuD forms a filament-dimer with itself. Within the past year, we have also identified and characterized several Umu homologs. One of these, called HumD, is found on the bacteriophage P1 genome. Interestingly, HumD encodes a protein of similar size and structure to the shorter, but mutagenically active, UmuD protein and not the larger inactive UmuD protein. Our studies revealed that HumD can functionally substitute for UmuD and promote mutagenesis together with E. coli UmuC. We have previously characterized the S. cerevisiae Rad30 protein. Studies by others recently demonstrated that Rad30 encodes DNA polymerase h. We have identified novel human and mouse homologs of pol h which we have called RAD30B. Human RAD30B is located on chromosome 18q21.1 in a region often implicated in the etiology of human cancers. In situ analysis of mouse Rad30b revealed that like many repair proteins, it is highly expressed in the testis.